positionChild method
Specify the child's origin relative to this origin.
Call this from your performLayout function to position each child. If you do not call this for a child, its position will remain unchanged. Children initially have their position set to (0,0), i.e. the top left of the RenderCustomMultiChildLayoutBox.
Implementation
void positionChild(Object childId, Offset offset) {
final RenderBox? child = _idToChild![childId];
assert(() {
if (child == null) {
throw FlutterError(
'The $this custom multichild layout delegate tried to position out a non-existent child:\n'
'There is no child with the id "$childId".',
);
}
return true;
}());
final MultiChildLayoutParentData childParentData = child!.parentData! as MultiChildLayoutParentData;
childParentData.offset = offset;
}